Make the music list, player list and inspiration list their own scenes

MusicListPanel, PlayersPanel (was Players) and InspirationList (was
ScrollContainer) each get their own scene and script. They report what
happens through signals, and the quiz wires them to the playlist, the
music player, the log and the popups. Every node keeps its place on screen.

Screenshots of every UI state are pixel-identical to before, the layout
test still matches the original baseline, and each part is tested on its
own as well as through the quiz.

music_list_panel.gd

@@ -0,0 +1,26 @@
+class_name MusicListPanel
+extends PanelContainer
+## The list of songs in the playlist, one row per song.
+
+## Emitted when the row of song number [param song_no] is clicked.
+signal song_clicked(song_no: int)
+
+var _song_list_object: PackedScene = preload("res://song_list_object.tscn")
+
+@onready
+var _rows: VBoxContainer = $ScrollContainer/MusicList
+
+
+## Shows the songs in the playlist again, so it shows their latest state.
+func refresh() -> void:
+    Settings.delete_children(_rows)
+    for s: SongObject in Playlist.get_playlist():
+        var new_song_list_object: SongListObject = _song_list_object.instantiate()
+        new_song_list_object.song_object = s
+        new_song_list_object.gui_input.connect(_on_row_input.bind(s.song_number))
+        _rows.add_child(new_song_list_object)
+
+
+func _on_row_input(event: InputEvent, song_no: int) -> void:
+    if event is InputEventMouseButton && event.pressed && event.button_index == MOUSE_BUTTON_LEFT:
+        song_clicked.emit(song_no)